This invention relates to A.C. motor control circuits in which both the amplitude and the frequency of the stator winding voltage of an A.C. motor are varied to vary the rotor speed thereof. One circuit of this type is disclosed in U.S. Pat. No. 3,878,445 which was issued to Edward E. Kirkham et al on Apr. 15, 1975 for an "A.C. MOTOR CONTROL APPARATUS AND METHOD".
In the Kirkham et al patent, an A.C. power supply voltage is rectified by a silicon controlled rectifier circuit whose D.C. output amplitude can be varied by varying the firing angle of the silicon controlled rectifiers. The D.C. output of the silicon controlled rectifier circuit is applied to an inverter which changes the D.C. voltage into an A.C. voltage. The frequency of the A.C. voltage can be varied by varying the frequency of an oscillator in the inverter circuit. The A.C. voltage is applied to the stator windings of an A.C. motor to drive the rotor thereof. Both the frequency and the amplitude of the A.C. voltage are selected in accordance with predetermined algorithms to produce the desired rotor speed and torque.
A pulse generator is coupled to the rotor of the A.C. motor and produces pulses whose frequency is proportional to the speed of the rotor. The pulse generator is coupled to a counter which is periodically interrogated to measure the actual speed and position of the rotor. The actual speed of the rotor is compared in a computer to the commanded speed thereof and an error signal is generated which indicates the difference between the actual speed of the rotor and the commanded speed thereof. The firing angle of the silicon controlled rectifiers is controlled to produce a D.C. output voltage which is related to the magnitude of the error signal by an empirically derived algorithm.